Polarization retaining fiber

ABSTRACT

PURPOSE: To form a polarization maintaining optical fiber which is long-sized and improves remarkably a polarization characteristic by forming a core of a circular or elliptical shape and forming a refractive index distribution of said core into a graded type shape.  
     CONSTITUTION: Holes  3′   a,    3′   b  for, stressing base materials are opened to a quartz bar  5  having 50 mm diameter and 150 mm length in the positions symmetrical with the center of the quartz bar and a hole  4 ′ for a core base material is opened at the center of the bar  5 , by means of an ultrasonic drill, then stressing base materials  3   a,    3   b  and a fiber core base material  4  are inserted into these holes. The stressing base materials consist of SiO&lt;SB&gt;2&lt;/SB&gt; glass doped with 10.5 mol % B&lt;SB&gt;2&lt;/SB&gt;O&lt;SB&gt;3&lt;/SB&gt; and 4.5 mol % GeO&lt;SB&gt;2&lt;/SB&gt;. The core base material is doped with 16 wt % GeO&lt;SB&gt;2&lt;/SB&gt;, has a refractive index distribution of a square function roughly in the radial direction. Said material has a ratio of five between the diameter of the clad and the diameter of the core. After the material  4  and the members  3   a,    3   b  are mounted to the bar  5 , the bar is drawn under the pressure reduced to about 10 &amp; sim; 100 Torr. The resulted polarization maintaining optical fiber is improved in crosstalk, i.e., the fiber having the core of a graded type refractive index distribution

BACKGROUND OF THE INVENTION

[0001] This invention relates to the fabrication of polarizationretaining single-mode (PRSM) optical fibers and more particularly to thefabrication of preforms from which fibers having elliptically-shapedcores can be drawn.

[0002] In many applications of single-mode optical fibers, eg.gyroscopes, sensors and the like, it is important that the propagatingoptical signal retain the polarization characteristics of the inputlight in the presence of external depolarizing perturbations. Thisrequires the waveguide to have an azimuthal asymmetry of the refractiveindex profile.

[0003] One of the first techniques employed for improving thepolarization performance of single-mode fibers was to distort thesymmetry of the core. A method of making this kind of PRSM fiber isdisclosed in U.S. Pat. No. 5,149,349, which is incorporated herein byreference. A PRSM optical fiber is formed by drawing a fiber from a drawblank having a glass core surrounded by cladding glass containingapertures that are diametrically opposed with respect to the core. Thefiber is drawn at such a rate and temperature that the apertures closeand the core becomes elliptically-shaped. In a preferred method ofmaking the draw blank, longitudinal grooves are formed on diametricallyopposed sides of a cylindrically-shaped core preform in which the glasscore is surrounded by the cladding glass. Glass particles are depositedon the outer surface of a glass tube, and the core preform is insertedinto the tube. The resultant assembly is heated to sinter or consolidatethe particles, whereby the tube is collapsed and fused to the groovedcore preform to form an assembly having longitudinal apertures onopposite sides of the core.

[0004] When the aperture-containing blank is drawn to form the PRSMfiber, the apertures close due to surface tension and the flow of moltenglass into the apertures. This flow causes the round core of the blankto become elongated in the direction of the apertures. The core aspectratio of the elliptically-shaped core is primarily determined by thespacing between the core and the apertures. As the spacing between thecore and the apertures in the draw blank decreases, the core of theresultant fiber becomes more elongated in cross-section, but the corecross-section tends to have long thin ends. If the spacing is too small,the core may break through the cladding region between core andapertures, resulting in the formation of a core the cross-section ofwhich is flattened with undesirable flared ends. By “ends” is meant thetips of the elongated core along the major axis of the core as observedin a plane perpendicular to the fiber longitudinal axis.

SUMMARY OF THE INVENTION

[0005] It is therefore an object of the invention to provide a method ofmaking PRSM optical fibers which overcomes the disadvantages of theprior art. A further object is to provide a PRSM fiber producing methodwhich is relatively simple to practice. Yet another object is to providea method for making elliptical core PRSM fibers having improved corecross-sectional shape.

[0006] In accordance with the present method, a polarization maintainingsingle-mode optical fiber is formed by initially forming an opticalfiber draw blank having a glass core region of refractive index n₁surrounded by a cladding glass region of refractive index n₂. Thecladding region includes apertures that are diametrically opposed withrespect to the core region and that are spaced from the core region. Thedraw blank has a region of low viscosity glass between the core regionand the apertures, the low viscosity glass region having a refractiveindex n₃ and a viscosity lower than that of the cladding glass region.An optical fiber is drawn from the draw blank at such a rate that theapertures close and the core becomes elliptical in cross-section.

[0007] Another aspect of the invention pertains to an optical fiberhaving an elliptically-shaped core of aspect ratio ρ₁, where ρ₁ equalsb₁/a₁, b₁ being the major axis radius and a₁ being the minor axis radiusof the core. Surrounding the core is a low viscosity glass region ofelliptically-shaped cross-sectional configuration having an aspect ratioρ₂ that is equal to b₂/a₂, b₂ being the major axis radius and a₂ beingthe minor axis radius of the low viscosity region. The aspect ratio ρ₂of the low viscosity region is less than ρ₁. A cladding glass regionsurrounds the low viscosity region. The viscosity of low viscosityregion is lower than that of the cladding glass region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a cross-sectional view of a preform from which anelliptical core PRSM fiber can be drawn.

[0009]FIG. 2 is a schematic diagram illustrating the drawing of a PRSMfiber from the preform of FIG. 1.

[0010]FIG. 3 is a cross-sectional view of a PRSM fiber produced by thepresent method.

[0011]FIGS. 4 and 5 are diagrams showing the major axis radius and theminor axis radius of the elliptical core and its surrounding lowviscosity region, respectively.

[0012]FIG. 6 illustrates the application of a coating of glass particlesto a mandrel.

[0013]FIG. 7 is a graph showing the core refracative index profile.

[0014]FIG. 8 is a schematic diagram illustrating the drawing of a rodfrom a consolidated core glass tube.

[0015]FIG. 9 is a schematic diagram illustrating the various dimensionsof a grooved -blank.

[0016]FIG. 10 is a cross-sectional view of an assembly wherein a groovedcore cane is disposed in a soot-coated cladding glass tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] It is to be noted that the drawings are illustrative and symbolicof the invention, and there is no intention to indicate scale orrelative proportions of the elements shown therein.

[0018] Draw blank 10 of FIG. 1, from which a PRSM fiber can be drawn,has core and cladding regions 11 and 12, respectively. The core andcladding regions may be formed of conventional materials employed in theformation of optical waveguides. The salient characteristics of thesematerials are that the refractive index n₁ of the core material must begreater than the refractive index n₂ of the cladding material and thatboth materials must exhibit low losses at the wavelength at which thewaveguide is intended to be operated. By way of example only, coreregion 11 may consist of pure silica or silica containing one or moredopants which increase the refractive index thereof. Region 12 mayconsist of pure silica, silica containing a lesser amount of refractiveindex increasing dopant than core region 11, or silica containing one ormore dopants, at least one of which is a refractive index loweringdopant such as B₂O₃ or fluorine. Although silica is a preferred baseglass because it exhibits low loss at useful wavelengths, base glassmaterials other than silica may be employed.

[0019] In accordance with the invention a region 14 of low viscosityrelative to that of cladding 12 is disposed between core 11 andapertures 13. The viscosity of low viscosity region 14 is preferablyclose to or slightly lower than that of core 11. This can beaccomplished, for example, by forming region 14 of silica doped withappropriate amounts of one or more viscosity lowering dopants such asB₂O₃, fluorine, P₂O₅, GeO₂ and the like.

[0020] The refractive index n₃ of region 14 should be equal to or lessthan n₂. The refractive index of region 14 can be made to be equal tothat of cladding 12 by, for example, forming region 14 of silica dopedwith appropriate amounts of one or more refractive index decreasingdopants such as B₂O₃ and fluorine and one or more refractive indexincreasing dopants such as P₂O₅, GeO₂ and the like.

[0021] The refractive index of region 14 can be made to be lower thanthat of cladding 12 by, for example, employing a silica cladding 12 andforming region 14 of silica doped with B₂O₃or fluorine or by dopingsilica with a refractive index increasing dopant such as P₂O₅, GeO₂ andthe like as well as a sufficient amount of B₂O₃ and/or fluorine toreduce the refractive index of the composite material to a value lessthan that of silica. Examples of elliptical core fibers having this typeof refractive index profile are disclosed in U.S. Pat. No. 5,482,525.

[0022] Apertures 13 extend longitudinally through blank 10 parallel tocore region 11. While apertures 13 are illustrated as being D-shapedin-cross-section, the cross-sectional configuration could be crescentshaped, circular, or the like. Any shape that results in the desiredcross-sectional elongation of the core during fiber draw is consideredto be suitable.

[0023] Referring to FIG. 2, draw blank 10 is placed in a conventionaldraw furnace where tractors 17 pull fiber 15 from the bottom portion ofblank 10 which is heated to draw temperature by heating elements 16. Thetendency for apertures 13 to close is a function of draw rate and glassviscosity. The viscosity of the draw blank root from which the fiber isdrawn depends upon furnace temperature and glass composition. If theviscosity of the heated portion of the blank is sufficiently low and thedraw rate is sufficiently low, apertures 13 will naturally close duringthe draw process. Since the apertures more readily close if they areevacuated, draw speed can be increased by affixing a vacuum attachment18 to the upper end of the blank.

[0024] As apertures 13 close, they are replaced by the surroundingglass. When glass at smaller radii than the apertures flows radiallyoutwardly into the apertures, core region 11 becomes elongated incross-section. The resultant PRSM fiber 15, the cross-section of whichincludes cladding 22, oblong core 21 and low viscosity region 23, isshown in FIG. 3. The ellipticity or aspect ratio of the elliptical coreis the ratio of its major dimension to its minor dimension in a planeperpendicular to the fiber axis (see FIG. 4). Cores of varying degreesof ellipticity can be made depending, inter alia, on the size ofapertures 13 and the spacing between those apertures and the core.

[0025] In accordance with the method of this invention, the shape ofelliptical core 21 is also a function of the viscosity of region 23.When the glass of draw blank 10 starts to flow, the flow of core glasstoward the apertures will be less restricted by the intervening glassbetween core region 11 and apertures 13 than was the case when thatintervening glass was silica. Thus, core region 11 can flow farthertoward the apertures before they close. Since the outer cladding 12 hasa relatively high viscosity, e.g. that of pure silica, the flow of thatglass into the apertures is very small; thus the core glass and the lowviscosity region 14 can flow more.

[0026] Consequently, the cross-sectional shape of core 21 is morebar-shaped (FIG. 4) as opposed to the usual shape with relativelypointed ends and a bulging middle. In the fiber resulting from thepresent process, the low viscosity region 14 assumes a cross-sectionalshape 23 (FIG. 3) having ends that are more pointed than the ends ofcore 21 and a middle section that bulges more than the middle of thecore. A comparison of FIGS. 4 and 5, which are relatively accuraterepresentations of the cross-sections of core 21 and region 23, revealsthat the aspect ratio b₁/a₁ of elliptical core 21 is greater than theaspect ratio b₂/a₂ of low viscosity region 23.

[0027] Apertures 13 must be parallel to the core and uniform incross-sectional area throughout the longitudinal axis of draw blank 10if fiber 15 is to have uniform properties throughout its length. Anyconventional technique that meets these requirements can be used forforming the apertures. UK Patent Application GB 2,192,289 teaches twotechniques for forming longitudinal holes in a preform on opposite sidesof the core:

[0028] (1) The holes can be drilled with a diamond drill.

[0029] (2) A core preform having opposed flattened sides is placed inthe center of a glass tube, and two glass rods are placed on oppositesides of of the core preform, leaving two opposed unfilled regionsbetween the core preform and the tube. The resultant assembly is drawnto reduce the diameter thereof and to cause the glass members to fusetogether to form an article that has a solid cross-section except fortwo opposed axe-head shaped holes that correspond to the unfilledregions.

[0030] An elliptical core PRSM fiber was made in accordance with themethod illustrated in FIGS. 6-10. A cylindrical mandrel 25 (FIG. 6) wasrotated and translated with respect to a flame hydrolysis burner 26 suchthat a stream 27 of glass particles or soot formed a porous coating 28on the mandrel. The composition of the soot stream 27 was initially SiO₂doped with 37 wt. % GeO₂, the concentration of GeO₂ decreasing withincreasing radius as shown in FIG. 7. The coated mandrel was removedfrom the lathe, and the mandrel was removed from the porous preform,thereby leaving a longitudinal aperture in the porous preform. Theporous preform was then dried and consolidated in accordance with theteachings of U.S. Pat. No. 4,125,388. The resultant consolidated preformor core blank 30 was inserted into the draw apparatus of FIG. 8 whereits tip was heated to drawing temperature by heating means 32, andvacuum connection 34 was affixed to its upper end. After the end of thepreform was stretched so that its aperture 35 was either very narrow orcompletely closed, the aperture was evacuated through fixture 34. As thelower end of the preform was pulled downwardly, and its diameterdecreased, the evacuated aperture 35 collapsed. The radius r of theresultant α-cane 31 was 6 mm. Its germania concentration profile isshown in FIG. 7.

[0031] A plurality of 90 cm sections were severed from the α-cane, andone of the sections was inserted into a lathe and coated with SiO₂ sootas described in conjunction with FIG. 6. The resultant composite preformwas consolidated at 1450° C. while a mixture of 94.3 volume percenthelium, 1.0 volume percent chlorine and 4.7 volume percent SiF₄ flowedupwardly through the muffle. In the resultant consolidated preform thediameter of the fluorine-doped silica layer was 13.4 mm, and the corediameter was about 6.2 mm.

[0032] The consolidated preform was inserted into a lathe and coatedwith SiO₂ soot which was consolidated in an atmosphere of chlorine andhelium to form a pure silica layer over the fluorine-doped silica layer.

[0033] Longitudinally-extending grooves were ground through the outersilica cladding layer on opposite sides of the core such that theyextended into the fluorine-doped low viscosity region. After thegrinding operation, the grooved β-blank (FIG. 9) was cleaned and rinsed.

[0034] The grooved β-blank was then inserted into a conventional drawfurnace of the type illustrated in FIG. 8 where it was stretched toreduce its diameter to about 7.3 mm. The resultant β-cane 40 (FIG. 10)includes core region 41, silica cladding layer 42 and low viscosityregion 43. Slots 44 extend longitudinally along β-cane 40 on oppositesides of core region 41.

[0035] Silica cladding tube 47 had inside and outside diameters of 7.5mm and 9.5 mm, respectively. An end of tube 47 was tapered inwardly andfused to a handle suitable for supporting assembly 52 in a consolidationfurnace. Grooved β-cane 40 was inserted into the end of tube 47 oppositethe tapered end until it contacted the tapered end. The end of tube 47into which preform 40 was inserted was tapered inwardly and fused to aglass rod. Tube 47 was then mounted in a lathe where it was rotated andtranslated with respect to a soot deposition burner where particles ofsilica soot were deposited thereon to build up porous coating 48,thereby forming assembly 52.

[0036] Assembly 52 was lowered into a consolidation furnace where it wassubjected to a drying gas mixture of chlorine and helium and thensintered to form optical fiber draw blank 10 of FIG. 1. As coating 48consolidated, it exerted a force radially inwardly on tube 47, therebyforcing that tube inwardly against preform 40. The original claddingregion 42 and tube 47 are completely fused together, and porous coating48 has become completely sintered and fused to tube 47, these layersforming cladding 12.

[0037] The resultant draw blank was inserted into a draw furnace, wherean optical fiber was then drawn from the preform.

[0038] A grooved β-blank made by the above-described process can becharacterized by the dimensions C, D, E and R of FIG. 10. Five differentgrooved β-blanks, which were made by the above-described method, wereemployed to form draw blanks that were drawn into optical fibers 1through 5, which are characterized in Table 1. In each of the groovedβ-blanks the radius R of the core region was about 3.5 mm. TABLE 1 FiberSoot wt. % Dimension Dimension Aspect No. Weight fluorine E (mm) C (mm)Ratio C1 N/A None 1.73 N/A 5.3 C2 N/A None 1.80 N/A 5.1 C3 N/A None 1.98N/A 5.8   1 160 g 1.6 1.53 11.99 6.2   2 160 g 1.6 1.53 11.99 6.3   3126 g 1.0 1.25 10.42 7.6   4  66 g 1.0 0.70  6.14 8.3   5  62 g 1.0 1.40 6.40 6.0

[0039] The dimension D of FIG. 9 was not measured but is related to the“soot weight” listed in Table 1. The soot weight is the weight of thesilica glass particles that are deposited to form low viscosity region43. The coating made up of those particles is then doped with fluorineand consolidated. A greater soot weight will result in a greaterthickness D.

[0040] The relatively high aspect ratio of fibers 3 and 4 indicates thata relatively small dimension E is preferred.

[0041] The three comparison fibers C1, C2 and C3 of Table 1 were made bya process similar to that by which fibers 1 through 5 were made, but nolow viscosity, fluorine-doped silica region 43 was employed. That is,the entire cladding from the core to the outer surface of the fiber wasformed of SiO₂. The aspect ratios of fibers C1, C2 and C3 are lower thanthe aspect ratios of fibers 1 through 5.

[0042] In the embodiment of FIG. 1 the low viscosity region 14 extendsfrom the core 11 and into the apertures 13. It is thought that someimprovement in aspect ratio would be achieved if region 14 did not quiteextend to apertures 13, but the effect on improved aspect ratio wouldnot be as great as the improved effect achieved by the embodiment ofFIG. 1.

1. A method of making an optical fiber comprising, forming an opticalfiber draw blank having a glass core of refractive index n₁ surroundedby a cladding glass layer of refractive index n₂, said blank havingapertures diametrically opposed with respect to said core and spacedfrom said core, said apertures extending longitudinally through saidblank and parallel to said core, said core glass having a viscosity andsaid cladding glass having a viscosity higher than the viscosity of saidcore glass, and said draw blank having low viscosity glass layer betweensaid core and said apertures, said low viscosity glass having aviscosity not greater than the viscosity of the viscosity of said coreglass and said low viscosity glass having a refractive index n₃, anddrawing an optical fiber from said draw blank at such a rate that saidcore glass and said low viscosity glass flow toward said apertures andsaid apertures close to form a core having an elliptical cross-sectionin which said low viscosity glass layer has a major axis radius b₂, aminor axis radius a₂, and an aspect ratio b₂/a₂ and the core glass has amajor axis radius b₁, a minor axis radius a₁ and an aspect ratio b₁/a₁,wherein the aspect ratio b₁/a₁ is greater than the aspect ratio b₂/a₂.2. A method according to claim 1 wherein the step of forming said drawblank further comprises, grinding longitudinally extending groovesthrough said cladding glass layer, said grooves extending into the lowviscosity glass layer; inserting said draw blank into a glass tube toform an assembly; and fusing said tube to said cladding glass layer toprovide an interface between said tube and said cladding layer and toprovide said longitudinally extending apertures.
 3. A method accordingto claim 1 wherein said low viscosity glass layer completely surroundssaid glass core.
 4. A method according to claim 1 wherein said lowviscosity glass layer is formed of SiO₂ containing dopants selected fromthe group consisting of B₂O₃, fluorine, P₂O₅ and GeO₂.
 5. A methodaccording to claim 1 wherein said cladding glass region consists ofSiO₂.
 6. A method of making an optical fiber comprising the steps of:forming a cylindrical assembly including a central core glass having aviscosity, a first layer of cladding glass surrounding said core, saidfirst layer of cladding glass having a viscosity not greater than theviscosity of the core glass, a second layer of cladding glasssurrounding said first layer, said second layer of cladding glass havinga viscosity being greater than that of said first cladding glass layer,a pair of longitudinal grooves extending along diametrically opposedsides of said second layer, a glass tube surrounding said second layer,and a layer of glass particles surrounding said tube; heating saidassembly to consolidate said particles, thereby exerting on said tube aradially inwardly directed force that causes said heated tube to shrinkonto and fuse to said second layer, thereby forming a draw blank havinglongitudinal apertures that are parallel to said core; and drawing thedraw blank to form an optical fiber having a solid cross-section.
 7. Anoptical fiber comprising a core having a viscosity, refractive index n₁and an elliptically-shaped cross-section having a major axis radius b₁,a minor axis radius a₁ and an aspect ratio ρ₁, wherein ρ₁ equals b₁/a₁;a low viscosity glass layer having a viscosity not greater than theviscosity of said core, an elliptically-shaped cross-section surroundingsaid core, said low viscosity layer having a major axis radius b₂ and aminor axis radius a₂, and an aspect ratio ρ₂, wherein ρ₂ equals b₂/a₂,wherein the aspect ratio ρ₁ is greater than the aspect ratio ρ₂; and acladding glass layer having a viscosity and a refractive index n₂surrounding said low viscosity layer, the viscosity of said lowviscosity layer being lower than the viscosity of said cladding glassregion.
 8. An optical fiber according to claim 7 wherein said lowviscosity layer is formed of SiO₂ containing dopants selected from thegroup consisting of B₂O₃, fluorine, P₂O₅ and GeO₂.
 9. An optical fiberaccording to claim 7 wherein said cladding glass layer consists of pureSiO₂.